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Database design is both an art and a science The science part comes in the form of adherence to certain rules and conditions, such as normalization (more about this later in the chapter) Database design is also an art, because you need to model relationships using your understanding of the realworld functioning of the organization You can formally define logical database design as the process of creating a model of the real world for the database, independent of an actual database system or other physical considerations Accuracy and completeness are the keys to this activity One of the best things about this stage is that it s easy to take a draft design, throw it away, and start again, or simply amend it.

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It s a whole lot easier to tinker at the design stage than to deal with the production headaches of an already implemented database that isn t designed well The logical design stage is sometimes broken up into a conceptual part and a logical part, but that s merely a distinction based on nomenclature The conceptual database design is usually a precursor for the logical design phase and involves the modeling of the information without reference to any underlying data model The logical design phase explicitly uses a specific data model, like the relational data model, for example you focus on the logical relationships involved in your conceptual design at this stage Logical design involves conceptually modeling the database and ensuring that data in the tables passes integrity checks and isn t redundant To satisfy these requirements, you need to implement data normalization principles, as you ll see shortly.

Entity-relationship modeling (ER modeling) is a widely used methodology for logically representing and analyzing the components of the business system, and it is commonly used to model the enterprise after the requirements analysis is completed The entity-relationship models are easy to construct, and their graphical emphasis makes them very easy to understand However, you can t build a real-life RDBMS using the entity-relationship model of an enterprise ER modeling s utility lies in designing databases, not implementing databases ER modeling can t form the basis of a high-level data-manipulation language like SQL, so the model that designers build using the ER modeling approach is translated to the relational model for implementation By converting the abstract entity-relationship design into a relational database schema, the relational model helps convert the entity-relationship design into a relational DBMS..

Tablespaces are not unique to Oracle. DB2 databases also have tablespaces, although Microsoft SQL Server databases don t use them. The tempdb database in a SQL Server database corresponds to the temporary tablespace in an Oracle database.

locked. (The length of the notification period is configurable; recall that at the beginning of the script I set it to start 14 days prior to the password expiring.)

You can create two basic types of tablespaces in an Oracle database, which differ by how they manage the database extents: locally managed and dictionary-managed tablespaces. Both types are described in the following section. And before you actually create a tablespace, you must be aware of two other important concepts: extent sizing and segment space management. These are discussed in the two subsequent sections.

Extents, as you know, are the basic unit of space allocation in Oracle databases, and dictionarymanaged tablespaces store extent information in the data dictionary. Locally managed tablespaces, on the other hand, manage extents by referring to the bitmaps kept in each physical data file header for all the blocks within that data file. Remember that a tablespace, which is a logical concept, is actually made up of one or more operating system files. For example, if a locally managed tablespace is made up of 128KB extents, each 128KB extent is represented by a bit in the extent bitmap for this file. The bitmaps indicate whether the blocks are free or occupied. If Oracle needs to allocate an extent to an object, the bitmap values are updated to show the latest status of the availability of data blocks. This takes the burden of free-space management from the data dictionary.

If you create your database with a locally managed System tablespace, any tablespaces that you create later on must also be locally managed.

The option to mix the compilation model within a source file is used to implement certain managed parts of the CRT. For other scenarios, I do not recommend using this feature, because it can cause access to uninitialized global or static variables. If you implement DLLs, you should especially avoid this approach. 12 provides more information on this topic.